A waste heat recovery system for a vehicle may recover thermal energy from engine exhaust, engine coolant, engine oil, and/or transmission fluid that would otherwise be dissipated to ambient surroundings. Waste heat recovery systems typically include a heat exchanger(s) to absorb heat from the engine coolant, engine oil, and/or transmission fluid, an evaporator to heat up the working fluid, an expander to produce mechanical work, a condenser to cool down the working fluid, and a pump to push the working fluid back to the heat exchanger(s). The mechanical work may then be converted energy via a generator for storage in a vehicle battery, for example.
One example waste heat recovery system is shown by Zhou in U.S. Patent Application Publication No. 2016/0201519. Therein, a vehicle is provided with an engine. The vehicle also has an expander, a condenser, and a pump in sequential fluid communication in a closed loop containing an evaporator configured to transfer heat between exhaust gas from the engine and a working fluid.
However, the inventors herein have recognized potential issues with such systems. As one example, vehicles typically include heat transfer systems in order to heat engine coolant, engine oil, and/or transmission fluid at engine cold start conditions to improve engine efficiency and reduce emissions. Such systems, which are separate from waste heat recovery systems, typically require at the minimum a heat exchanger, a heating loop, and several control valves, which increase system cost and packaging space.
In one example, the issues described above may be addressed by a method comprising, responsive to a first condition, expanding a working fluid of a waste heat recovery system via an expander to generate electricity, and responsive to a second condition, compressing the working fluid of the waste heat recovery system via the expander to transfer heat to an engine. In this way, the expander of the waste heat recovery system may be differentially operated to either convert waste heat to energy (e.g., electricity) for storage in a vehicle battery, for example, or to pump the working fluid to transfer absorbed waste heat to the engine via one or more engine component heat exchangers, for example.
As one example, working fluid in the waste heat recovery system may be heated via an evaporator in thermal communication with exhaust gas from the engine. Then, the heated working fluid is directed to the expander via either a first flow path or a second flow path. Directing the working fluid through the first flow path causes the working fluid to expand across the expander, and this expansion drives rotation of the expander in a first direction, which in turn drives a generator to generate the electricity. Directing the working fluid through the second flow path causes the working fluid to be compressed at the expander via rotation of the expander in a second direction, driven by a motor. The compressed working fluid is pressurized by the compression to a sufficient degree to drive the working fluid to flow through the one or more engine component heat exchangers. By doing so, both engine heating and waste heat energy generation may be provided by a single system, reducing or eliminating the need for engine cooling system valves, heat exchangers, and/or other components. Further, by flowing the working fluid directly from the expander to the one or more engine component heat exchangers via the expander rotating in the second direction, heat loss at other components of the system (e.g., condenser, pump, and associated conduits) may be avoided.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.